1. Field of the Invention
The invention is directed to an improved fuel injection system for an internal combustion engine.
2. Description of the Prior Art
One fuel injection system known from German Patent Disclosure DE 101 19 984 A has one high-pressure fuel pump and one fuel injection valve, communicating with it, for each cylinder of the internal combustion engine. The high-pressure fuel pump has a pump piston, which is driven at least indirectly by the engine in a reciprocating motion and which defines a pump work chamber. The fuel injection valve has an injection valve member, by which at least one injection opening is controlled. The injection valve member is urged in an opening direction by the pressure prevailing in a pressure chamber communicating with the pump work chamber and is movable counter to a closing force in the opening direction for uncovering the at least one injection opening. An electrically actuated control valve is provided, by which a communication of the pump work chamber with a relief region is controlled at least indirectly. A movable control piston is furthermore provided, which acts at least indirectly on the injection valve member in the closing direction and which is acted upon, on its side remote from the injection valve member, by the pressure prevailing in the pump work chamber. The closing force on the injection valve member is generated by a closing spring, and the control piston acts on a support of the closing spring. In the pumping stroke of the pump piston, the control valve is closed at a defined instant, so that the pump work chamber is disconnected from the relief region, and high pressure builds up in the pump work chamber. In the pressure chamber, the same high pressure prevails as in the pump work chamber, and if a higher force on the injection valve member is exerted by this pressure than by the closing spring, then the injection valve member moves in the opening direction and uncovers the at least one injection opening, so that fuel is injected. Upon a further pressure increase in the pump work chamber, the control piston is displaced counter to the force of the closing spring, until it comes into contact with a stop, as a result of which the force exerted by the closing spring on the injection valve member is increased, and the injection valve member is moved into its closing position, thus ending the fuel injection. During the fuel injection, only a slight fuel quantity is injected in the form of a preinjection. After that, the pressure in the pump work chamber rises further, and when a greater pressure on the injection valve member is exerted by the pressure than by the closing spring, the injection valve member then moves in the opening direction again and uncovers the at least one injection opening, so that fuel is once again injected. During this fuel injection, a larger fuel quantity is injected as a main injection, to meet the demand of the engine. For terminating the main injection, the control valve is opened, so that the pump work chamber communicates with the relief region, and a high pressure no longer prevails in it; the injection valve member is moved in its closing direction by the closing spring, and the control piston returns to its outset position. The preinjection, and in particular its timing and the preinjection quantity, is defined structurally in this fuel injection system by the design of the control piston, its possible stroke, and the closing spring, so that the preinjection and the time interval until the next main injection, as well as the pressure at which the main injection begins, cannot be adapted flexibly as a function of various operating conditions of the engine. Another disadvantage of the known fuel injection system is that exact control of very slight fuel injection quantities is made more difficult. Moreover, closure of the fuel injection valve, or in other words a termination of the fuel injection, does not occur until at a low pressure in the pump work chamber, thus increasing the pollutant emissions from the engine. Moreover, the major pressure gradients that occur upon the opening and closure of the control valve cause a loud noise in the fuel injection system and considerable pressure fluctuations in the relief region.